1. Field of the Invention
The present invention concerns a method as well as an image evaluation system for preparation of medical 2D or 3D data, in particular of 2D or 3D image data acquired by means of computed tomography.
2. Description of the Prior Art
2D and/or 3D data in the medical field are acquired as the result of radiographic methods such as, for example, computed tomography, mammography, angiography, x-ray inspection technology or comparable methods. In these methods, the representation of the attenuation of an x-ray along its path from the radiation source (x-ray source) to the detector system (x-ray detector) ensues in a projection image. This attenuation is caused by the irradiated materials along the beam path, such that the attenuation can also be understood as a linear integral across the coefficients of all volume elements (voxels) along the beam path. In x-ray computed tomography (CT), it is possible by reconstruction methods to back-calculate the attenuation coefficients μ of the individual voxels from the projected attenuation data, and therefore to achieve a significantly more sensitive examination compared to consideration of the individual projection images.
Instead of the attenuation coefficients μ, a value that is normalized to the attenuation coefficients of water (known as the CT count) is used to show the attenuation distribution. This is calculated from a current attenuation coefficient μ (determined by measurement) according to the following equation:
  C  =      1000    *                  μ        -                  µ                                    H              2                        ⁢            O                                      µ                              H            2                    ⁢          O                      ⁢          (      HU      )      with the CT count C in Hounsfield units (HU). A value CH2O=0 HU results for water and a value CL=−1000 HU results for air. Since both representations can be transformed into one another, and thus are equivalent, in the following the general terms attenuation value or attenuation coefficient designate both the attenuation coefficients μ and the CT value.
Modern x-ray computed tomography (CT) apparatuses are used for the acquisition, evaluation and presentation of the three-dimensional attenuation distribution in the medical field. A CT apparatus typically has a radiation source that directs a collimated, pyramidal or fan-shaped radiation beam through the examination subject (for example a patient) onto a detector system constructed from multiple detector elements. Depending on the design of the CT apparatus, the radiation source and the detector system are mounted, for example, on a gantry or on a C-arm that can be rotated around a system axis with an angle φ. Furthermore, a support device for the examination subject is provided that can be shifted or moved along the system axis. During the acquisition, each detector element of the detector system that is struck by the radiation produces a signal that represents a measure of the total transparency of the examination subject for the radiation emanating from the radiation source on its path to the detector system, i.e., the corresponding radiation attenuation. The set of output signals of the detector elements of the detector system that is acquired for a specific position of the radiation source is designated as a projection. The position, starting from which the radiation beam passes through the examination subject, is continuously altered as a result of the rotation of the gantry/C-arm. A scan thereby encompasses a number of projections that are acquired at various positions of the gantry/C-arm and/or various positions of the support device. Differentiation is made between sequential scan methods and spiral scan methods.
A two-dimensional slice image of a slice of the examination subject can be reconstructed on the basis of the data set generated in a scan. The quantity and quality of the measurement data acquired during a scan depend on the employed detector system. Multiple slices can be acquired simultaneously with a detector system that has an array composed of multiple rows and columns of detector elements. Detector systems with 256 or more rows are known today. The generated scan data and/or image data are typically prepared and visualized with an image evaluation system (for example a post-processing workstation).